DESCRIPTION (Applicant's Abstract): It is clear that the c-myb protooncogene plays a crucial role during hematopoiesis. In each hematopoietic lineage, c-myb is abundantly expressed at the immature stages of differentiation and is turned off at a relatively late time during the differentiation process. However, virtually nothing is known about what role c-myb plays during hematopoietic maturation, how that role is mediated or what the signaling pathways are that regulate c-myb expression. Mice that are homozygous null at the c-myb locus die at day fifteen during embryogenesis from a severe anemia. This finding graphically demonstrated the significance of c-myb during hematopoiesis but has precluded study of c-myb activity at the later stages of differentiation. The lack of a tractable genetic system that will allow mutation of c-myb during the later stages of hematopoietic maturation has been a major impediment to understanding the role of c-myb during hematopoiesis. During T-cell development in the thymus, c-myb is expressed in CD4+CD8+ double positive thymocytes but is expressed at essentially undetectable levels in CD4+ and CD8+ single positive cells. In the periphery, c-myb is not expressed in resting T-cells, but is expressed in proliferating T-cells in late Gi/early S-phase of the cell cycle. To begin to understand the role played by c-myb during T-cell development we have produced mice that carry a c-myb allele targeted with loxP sites for deletion by the Cre recombinase. By breeding these mice to available mouse strains that direct Cre expression either to the early stages of T-cell development in the thymus or in an inducible fashion, we will define the role played by c-myb during T-cell development and the activation of effector functions. In addition, these mice will be crucial to the field and will allow one to begin to gain insight into c-myb function during hematopoiesis as well as in other systems where c-myb function remains poorly understood. The goals of this proposal are: 1) to determine at what stage c-myb expression is essential for T-cell development and function, 2) to determine the consequences of inappropriate c-myb expression to T-cell development and function and 3) to identify c-myb functional domains required to drive T-cell development in the thymus.